I am not sure if I understand exactly what you are talking about.
When our house was about 4 yrs old (we built it), we ripped out everything the biulder installed, including 2 outside units, 2 upstairs firnaces and ductwork. We spent about $20,000 but we've had no issues.
Everything is still under a warranty.
If I were you, I'd check into how much a total replacement would be. Sometimes you are better off just biting the bullet and investing once vs getting something fixed over and over at the tune of $600. Also, make sure you know your service man and trust him.
Our service guy bought the last house we built so we always tell him, we know where you live!! We are friends.